have an affair vs step out

have an affair

verb
  • To engage in a romantic or sexual relationship while being married. 

step out

verb
  • To date, to be in a romantic relationship. 

  • To exit a place on foot, often for a short time. 

  • To increase the length, but not the rapidity, of the step. 

How often have the words have an affair and step out occurred in a corpus of books? (source: Google Ngram Viewer )